The present invention relates to near field antenna measurement systems generally and, more particularly, to near field measurement systems that allow the signal from a monitoring antenna in the near field to be processed so that it approximates a signal that would be received if the monitoring antenna were located in the far field.
While the present invention is described in connection with near field antenna monitors for microwave landing systems (MLS) for aircraft, it will be understood by those skilled in the art that it has other useful applications as well.
In part an MLS is concerned with angle guidance for aircraft, which is accomplished through the use of two electronically scanned microwave antenna arrays--one for azimuth and the other for elevation. Such an MLS system is described in U.S. Pat. No. 3,999,182 of Moeller et al. This system uses an antenna made up of a plurality of radiating elements spaced along a linear axis to the left and right of a center element. Each element on the left and right is fed r.f. energy from a common feed line through individual electronically variable phase shifters. In order to produce a specific radiation pattern, e.g. a beam focused at a remote point, each element receives a precisely determined portion of the power from the feed line with a particular phase. By varying the phase electronically with the phase shifters, the beam can be made to scan TO and FRO at a particular rate.
Since this system is critical for aircraft safety, each antenna is continuously monitored in the field in real time to assess the quality of the guidance angle data. The critical monitored parameter is the time between the centers of the TO and FRO beams, which is the guidance angle information. Preferably, the beam is monitored in the far field, i.e. at the distance from the antenna where the beam is focused. This distance from the antenna is at least 2D.sup.2 /.lambda., wherein D=the aperture size or width of the antenna ray and .lambda.=the wavelength of the transmitted signal, all in feet. For a 1 degree beam width antenna, this distance is approximately 1400 feet.
U.S. Pat. No. 4,926,186 of Kelly et al. described a scanning phased array monitor which uses a single receiver located in the far field at an angle to the main axis of the antenna or an integral waveguide monitor fixed to the antenna. In the Kelly type arrangement, far field signals from the monitor are converted into in-phase, I, and quadrature phase, Q, signals that are detected and sampled at non-uniform intervals. These samples are processed according to Fast Fourier Transforms to determine the amplitude and phase of the phase shifter outputs. The output signals are compared to reference values. Any deviation from the reference values indicates which element of the antenna array is malfunctioning. This monitor, however, is not a guidance angle monitor.
In many cases, it is impractical to locate guidance angle monitors in the far field. For example, in the area from the antenna to a point 1400 feet away there may be physical obstructions, e.g. buildings and airport traffic, that will disturb the beam. Also, there may be radio frequency interference sources, such as radio signals from taxiing aircraft.
Locating the monitor in the near field where the transmitted beam is unfocused can also result in unacceptable performance, primarily due to systematic phase variation which is related to the path difference variation of the individual array elements to a field monitor antenna in the near field. While somewhat acceptable results can be obtained with monitors located about 240 feet from the antenna (i.e. 1/3 D.sup.2 /.lambda., there is considerable room for improvement.
The approach of making antenna measurements in the near field also results in significant degradation in system integrity and continuity of service, which are critical requirements for zero visibility landing capability specified by FAA regulations. The integrity is diminished because monitoring accuracy is degraded due to defocusing of the beam at that distance. In particular, some angle guidance errors go undetected by the monitor in the near field, e.g., a 90.degree. phase error at close range may look like a 180.degree. phase error and cause no monitor error, while the actual radiated signal-in-space has an error.
Continuity is lost because, in some cases, the MLS signals is within tolerance limits, but the monitor may indicate that it is outside the tolerance limits. In effect the near field monitor may cause false alarms when certain antenna components fail. The transmitting ground equipment is designed to operate with a specified number of component failures so as to increase the continuity of system service. However, with the antenna in the near field the loss of an element in the phased array may cause an out-of-tolerance monitor error. As a result the system will automatically shut down. Thus, there are unnecessary interruptions in service. Also, in some cases, even a distance of 240 feet is impractical and it would be desirable to be able to locate the monitor closer, say perhaps as close as 80-90 feet, and still be able to obtain acceptable results.
Integral monitors as mentioned in the Kelly et al. patent are fixed to the array. As a result, if there is mechanical motion of the array or there is ice or snow on the radome which affects the angle guidance signal, the integral monitor will not detect it. The field monitor is sensitive to all effects, including mechanical motion and radome effects.
In copending U.S. application Ser. No. 07/591,698 of Feldman et al., there is described a near field monitor that achieves acceptable monitoring of the performance of an antenna ray. According to this application a monitor, in the form of one receiving antenna, is located between 80 and 90 feet from the antenna. The signals from the receiving antenna are processed in terms of amplitude and phase, and are combined so that the signal aproximates that which would be received if it were in the far field. In particular the signals are resolved into a series of I and Q samples which are stored in a buffer memory. A subset of these stored samples is multiplied by separate weighing factors and then combined according to ##EQU1## and where WI.sub.m, WQ.sub.m are the weighing factors for the I.sub.m and Q.sub.m components, respectively and M is the total number of weights. The process is a Finite Impulse Response (FIR) digital filter process.